Tomosynthesis combines digital image capture and processing with some portion of the source/detector motion available in 3-D tomography to provide a measure of depth information from an imaged subject. By contrast with computed tomography (CT), digital tomosynthesis uses a small rotation angle, typically between 20° and 40°, with images acquired by varying the orientations of the x-ray tube relative to the patient and detector with a small number of discrete slices/exposures (e.g., 10 exposures). This is usually accomplished by either moving both the detector and x-ray source or by fixing the position of the detector and moving the x-ray source. In applications where the detector is fixed, multiple spatially distributed X-ray sources may be used or one or more movable sources may be discretely displaced and fired in various imaging patterns or trajectories.
The set of image data that is acquired, which is partial with regard to full volume image information, is digitally processed to yield an image similar to tomography but with a limited depth of field. Depth data is reconstructed from the captured projections in the form of a number of slices through the patient anatomy, with the best resolution from each slice taken parallel to the detector plane. A consequence of limited angular scanning for imaging used to reconstruct a 3-D object is that the in depth resolution is characteristically lower than the in-plane resolution of the reconstructed object. Since the image is digitally generated and represented, various processing techniques can be used to generate and present a series of slices at different depths and with different thicknesses reconstructed from the same image acquisition, thereby saving time and reducing radiation exposure.
Because the tomosynthesis data that is acquired is incomplete in terms of full three dimensions of data content, tomosynthesis does not offer the narrow slice widths and enhanced depth definition that CT offers. However, tomosynthesis provides high in-plane resolution and is advantaged over 2-D radiography by providing a measure of depth detail that is not otherwise available with conventional radiography.
A tomosynthesis imaging apparatus may have any of a number of source-detector arrangements for image acquisition. In a distributed array configuration, an array of X-ray sources may be disposed in a generally circular or other geometric distribution. Such a distribution may surround a central X-ray source that may include a standard radiography source, or the distribution may be arranged as a linear or curved path. Distributions of carbon nanotube (CNT) cathode X-ray sources may be arranged to provide tomosynthesis imaging without the need to reposition either the radiation source or the detector. Reference is made to an article by Je Hwang Ryu, Jung Su Kang, and Kyu Chang Park, entitled “Carbon Nanotube Electron Emitter for X-ray Imaging” in Materials, 2012, 5, 2353-2359 which is incorporated herein by reference in its entirety for nonessential background information. Reference is also made to U.S. Pat. No. 8,172,633 to Park et al., filed Apr. 4, 2007; U.S. Patent Application Publication No. 2011/0003109 by Slinker et al., filed Jul. 1, 2009; and U.S. Pat. No. 7,505,562 to Dinca et al., filed Apr. 19, 2007, which are incorporated herein by reference in their entireties.
One difficulty with distributed source arrangements relates to the need for appropriate collimation of emitted radiation. Among its functions, collimation controls the spread of radiographic energy so that it is appropriately directed to the anatomy of interest and so that it does not extend beyond the outer edges of an imaging detector. Collimation also helps to reduce scattering of radiographic energy. With CNT or other types of small x-ray sources in an array, collimation techniques present particular challenges. One set of problems relate to dimensional constraints. Because the spacing between such x-ray sources is typically small, it can be difficult to effectively bound the radiation energy emitted from any individual source. Crosstalk can occur, making it difficult to clearly define edges of the radiation field. Still other complexity relates to identifying the radiation field for imaging from each source. With conventional radiography sources, the problem is readily solved: a light source that is coupled to the radiography source can be used to outline or otherwise highlight a radiation field represented by visible light, by adjusting the collimator edges. However, it can be impractical or impossible to provide a corresponding dual-use arrangement using collimator openings provided for CNT and other types of distributed array sources.
Thus, it can be seen that although there can be advantages in using a distributed array of x-ray sources for tomography and other types of depth imaging, existing collimation strategies fall short of what is needed to more effectively collimate the emitted radiation and generate useful image projections and reconstructions.